The present invention relates to improvements in and relating to the preparation of polypropylene based polymers, in particular to the preparation of polypropylene polymers having excellent impact strength as well as high resistance to stress whitening.
Polypropylene has unique properties such as low density, excellent chemical resistance and rigidity. However, certain polypropylene polymers, e.g. homopolypropylene and polypropylene random copolymers (RACOs), have poor impact resistance especially at low temperatures. This has led to the development of a large number of propylene based polymers in which polypropylene polymers are modified by blending with elastomers, e.g. ethylene-propylene rubbers, in which the elastomer forms a dispersed phase in a polypropylene matrix thereby improving impact strength. Heterophasic polypropylene copolymers (HECOs), i.e. polymers containing a propylene polymer matrix and an elastomer, are one example of such materials.
Although having good impact resistance, heterophasic polypropylene copolymers are susceptible to stress whitening when exposed to mechanical stresses. When damaged (e.g. when bent or subjected to impact) the optical appearance of the polymer alters, i.e. this becomes opaque. Although stress whitening may have no effect on the geometrical and/or mechanical properties of the polymer, this nevertheless limits the use of such materials in cases where the appearance of the polymer product is important, e.g. in the production of toys, household and technical appliances, transport and storage boxes, etc. Since stress whitening can also lead to surface damage, it is also generally undesirable to use such polymer products for packaging of food or medical products where it is important that packaging should be kept sterile.
Stress whitening associated with heterophasic polypropylene copolymers can be reduced by further inclusion within the polypropylene material of a plastomer. Typically, a plastomer may be dispersed in the polypropylene material as the result of a blending process.
When blending any polypropylene based polymer (e.g. a polypropylene homo- or copolymer, or a heterophasic polypropylene copolymer) with an elastomeric polymer (e.g. a plastomer), it is generally considered necessary to ensure that the density, weight average molecular weight (Mw) and/or MFR of the elastomeric component (e.g the plastomer) is matched to that of the polypropylene component to ensure adequate homogenization of the resulting blend. This may, for example, be achieved by prior blending of the polypropylene component with a suitable plastomer.
Typically, polypropylene blends are prepared by blending or compounding of separate polymer components, e.g. a propylene polymer material and a plastomer, produced in different polymer plants. As a result, transport and handling costs are high. An additional compounding or extrusion step is also necessary to produce the final polymer blend.
In most polymer plants (e.g. those producing polypropylene), polymerization reactors, supporting systems and extruders are designed to have identical production capacities. This may present problems when an attempt is made to add a second polymer component (e.g. a plastomer) immediately prior to extrusionxe2x80x94due to the limited capacity of the extruder it is generally necessary to reduce the rate of polymer production within the plant. Clearly, this is undesirable.
Alternatively, so-called xe2x80x9creactor blendsxe2x80x9d can be produced by means of a cascade polymerization process in which the same or different catalyst systems are employed to produce different polymers, typically in two or more separate reactors connected in series. Multi-stage processes in which different catalyst systems are employed in sequential polymerization stages are described, for example, in EP-A-763553 (Mitsui) and WO 96/02583 (Montell). In a cascade process in which different catalysts are used in sequential reactors, the catalyst from the preceding reactor remains active following discharge of the reaction mixture into the next reactor. Inevitably, this results in a lack of control over the characteristics of the final polymer product. For example, in the case where a Ziegler-Natta catalyst used in a first polymerization stage remains active during a second stage effected in the presence of a different catalyst system, a large proportion of the final polymer material will comprise a high molecular weight polymer having a broad molecular weight distribution. This can lead to undesirable polymer properties.
WO 96/11218 (Montell) describes a multi-stage polymerization process in which a first catalyst is deactivated prior to the introduction of a second catalyst system. Specifically, the process described in WO 96/11218 comprises a first stage in which a propylene polymer is produced in the presence of a first titanium or vanadium catalyst, a second stage in which the catalyst is deactivated, and a third stage in which polymerization is continued in the presence of a second metallocene catalyst. Such a cascade process is believed to result in good homogenization of the resulting polymer blend. However, the need to deactivate the first catalyst before the polymer particles can be impregnated with the second catalyst makes this process unnecessarily complex and not cost effective. A further disadvantage of this process is that the second catalyst is relatively quickly flushed out of the reactor as a result of the high throughput of polymer material into the third stage of the polymerization process.
Contrary to current thinking, we have now found that the demands of the step of homogenization of a polypropylene based polymer and an elastomeric polymer, e.g. a plastomer, are not essential to provide a polypropylene material having the desired properties of high impact resistance, resistance to stress whitening, etc. As a result, preparation of the individual polymer materials can be effected in separate polymerization reactors run in parallel followed by simple blending (e.g. compounding) of the resulting polymer components. This offers significant advantages in terms of costs, process operability, optimization of desired polymer properties, etc. Surprisingly, we have found that adequate homogenization can readily be achieved by co-extrusion of the separately produced polymer materials.
Thus, viewed from one aspect, the invention provides a process for the production of a propylene based polymer, which process comprises:
(a) a first polymerization stage comprising homopolymerizing propylene or copolymerizing propylene and at least one xcex1-olefin in the presence of an xcex1-olefin polymerization catalyst whereby to produce a polypropylene component;
(b) a second polymerization stage comprising copolymerizing ethylene and at least one xcex1-olefin in the presence of an xcex1-olefin polymerization catalyst whereby to produce an ethylene/xcex1-olefin copolymer component; and
(c) blending the polymer components produced in steps (a) and (b) whereby to produce a polymer blend,
wherein said first and second polymerization stages are effected in separate polymerization reactors connected in parallel.
Preferably, blending may be effected by co-extrusion of the polymer components produced in steps (a) and (b). Alternatively, the process of the invention may comprise the further step of extruding the polymer mixture following simple blending of the polymer components whereby to produce a substantially homogenous polymer.
In addition to the advantages outlined above, the process herein described effectively permits an increase in the duration of the second polymerization stage (since the total output from that stage is the elastomeric, e.g. plastomeric, component). The ability to increase the residence time in the reactor (e.g. a gas phase reactor) increases the productivity of any catalyst used. This also permits increased flexibility of the process in terms of altering the desired mechanical properties of the final polymer blend.
Polymers produced by a process according to the invention exhibit good impact properties, enhanced stress whitening resistance, and good optical properties (haze). Such polymers form a further aspect of the invention.